This invention relates generally to Micro-Electro-Mechanical-Systems (MEMS). More particularly, it is related to a novel class of vertical combdrive devices serving as rotating actuators and/or position sensors and methods for operating any combdrive.
The advent of silicon fabrication technologies has made possible a line of integrated devices in which micro-actuators and micro-mechanical structures are fabricated using processing technology similar to that used in the integrated-circuit industry. These integrated actuators have been employed in a variety of applications, such as fiber-optic switching, optical tracking for applications such as free-space communications, inertial sensors, and magnetic disk drives. They offer small size, low cost, high reliability, and superior performance. Furthermore, micro-machined structures may be integrated with Integrated Circuits (ICs) fabricated on the same substrate.
Various actuation methods can be employed in these integrated actuators, including electrostatic, electromagnetic, thermal, and thermo-pneumatic means. Electrostatic actuation becomes particularly attractive on a small size scale, since the electrostatic force increases as the gap between two charged elements decreases. Combdrive electrodes are widely used for generating electrostatic driving forces.
It is often desirable to create out-of-plane actuation of various microstructures, such as rotation of mirrors about an axis parallel to a substrate. These rotating mirrors can be used individually or in array form for applications such as adaptive optics, visual displays, or fiber-optic switching. Vertical combdrive actuators provide rotational motion or translational motion perpendicular to a substrate. A micromachined electrostatic vertical actuator is disclosed in U.S. Pat. No. 5,969,848, issued to Lee et al. The device of Lee et al. contains a set of vertical combdrives, with each drive capable of deflecting one edge of a square mirror. By relying on an asymmetric distribution of electrical fields when a bias voltage is applied between stationary and movable comb fingers, the device of Lee et al. allows a small vertical (i.e. out of the plane of the comb fingers) motion of each mirror edge, at most 1.5 xcexcm.
Larger movements and more simplified fabrication techniques are provided by staggered vertical combdrives, in which the stationary and moving combdrives are positioned parallel to one another, but with the plane of the moving comb above the plane of the stationary comb. The stationary comb fingers are an integral part of the substrate, while the moving comb is fixed to the substrate only through flexures. Applying a voltage between the two comb layers causes the moving comb teeth to be attracted to the stationary teeth and move to increase the overlap area, thereby exerting a force on the moving comb. Conventional fabrication techniques for vertical combdrives using standard photolithography processes require multiple steps for patterning the comb fingers. First, one set of comb teeth is fabricated on a first wafer layer. A second wafer layer is then bonded on top of the first wafer layer, followed by patterning and etching of a second layer to form the second set of comb teeth. The two wafer layers must be aligned to a very high precision; typical applications require comb fingers of 2 xcexcm wide with a 6 xcexcm separation distance, so that adjacent overlapped fingers are separated by only 2 xcexcm. Vertical combdrives fabricated using this technique are prone to alignment problems. The steppers used to align the individual die on a wafer typically have a lateral resolution of xc2x10.25 xcexcm. This resolution places a lower limit on the gap between adjacent comb fingers of about 2 xcexcm. Because two adjacent fingers are at different potentials during operation, they cannot contact each other. At high actuation voltages, errors in alignment of the fingers can cause sideways motion and instability in the combdrive. As a result, conventional fabrication techniques typically have low production yields.
FIGS. 1A-1B depict a prior art rotating actuator employing a two-layer vertical combdrive. FIG. 1A shows rotating actuator 100 in a nominal state. A plurality of movable comb fingers 10, extending from a first micro-machined structure 11, are suspended above a plurality of stationary comb fingers 12, which extend from a second micro-machined structure 13. A rotating element 14, attached to a flexure 15, is mechanically engaged with first micro-machined structure 11 and therefore movable comb fingers 10. Rotating element 14 may carry a reflective surface so as to provide a scanning mirror for a given application. It is worth noting that stationary comb fingers 12 and movable comb fingers 10 are fabricated in two different layers of a substrate (not shown in FIG. 1A). FIG. 1B depicts a rotating state of rotating actuator 100 of FIG. 1A. The rotation can be generated by an electrostatic means, e.g., by applying a voltage between stationary comb fingers 12 and movable comb fingers 10. The capacitance between movable comb fingers 10 and stationary comb fingers 12 may be measured and resolved to determine and control the angular position of movable comb fingers 10.
In a combdrive actuator, it is desirable for the angular position of the movable comb fingers to vary with the applied voltage in a linear fashion; and it is also desirable for the stationary comb fingers and movable comb fingers to be aligned with respect to each other in a precise lateral alignment. This is owing to the fact that if the stationary and movable comb fingers are not well aligned, such that each of the movable comb fingers is centered within the gap between its respective neighboring stationary fingers, there arises a net lateral force upon application of a voltage between the stationary and movable comb fingers. Such a lateral force can cause non-linear and unstable behaviors in the motion of movable comb fingers. For example, sufficient lateral force can cause the movable comb fingers to snap into contact with the stationary comb fingers.
In the prior art combdrive system of FIGS. 1A-1B, however, because stationary comb fingers 12 and movable comb fingers 10 are not coplanar and therefore not substantially engaged in their initial positions, the motion of the combdrive thus constructed is significantly nonlinear, unless a sufficient force is exerted on the combdrive to engage stationary comb fingers 12 and movable comb fingers 10. Moreover, precise lateral alignment between stationary comb fingers 12 and movable comb fingers 10 is also inherently difficult to achieve in the above prior art combdrive system, because stationary comb fingers 12 and movable comb fingers 10 are fabricated in two different layers of the substrate. This can further result in non-linear and unstable behavior
For example, in the prior art combdrive system of Conant et al. titled xe2x80x9cStaggered Torsional Electrostatic Combdrive Micromirrorxe2x80x9d, U.S. pending application serial number 09/584,835, a set of stationary combdrives is fabricated in one lithographic masking and etching step, and a set of moving combdrives is fabricated in a subsequent lithographic masking and etching step. A precise lateral alignment of these two sets of combdrives would dictate aligning the second lithographic masking step to the pattern of the first lithographic masking step during the manufacturing process.
Hence, any error in this alignment translates directly to a lateral misalignment between the movable and stationary combdrives, which results in nonlinear and unstable behaviors in the combdrive actuator thus produced.
Hence, there is a need in the art for a new type of rotating combdrive actuators and position sensors that overcome the shortcomings of the prior art systems, while providing a wide range of angular rotation along with versatile actuation and sensing capabilities.
The disadvantages associated with the prior art are overcome by a rotating device having one or more comb structures and biasing element to apply torsion force thereto and in response to position sensing of the rotating device. The device generally comprises a plurality of first comb fingers that interdigitate with a plurality of second comb fingers. In one embodiment, both pluralities may be fabricated from a single layer of a substrate such that they are self-aligned. The design of the combdrive device is such that in a nominal state, the two sets of comb fingers are substantially interdigitated according to a predetermined engagement. A rotating element, attached to a rotatable flexure disposed along an axis, is mechanically engaged with the first comb fingers. A biasing element (e.g., a magnetic material) is attached to the rotating element. When subject to a biasing force (e.g., a magnetic force), the biasing element causes the rotating element along with the first comb fingers to undergo a controlled angular displacement from the initial engagement. In one embodiment of the present invention, the combdrive device serves as a rotating actuator. This is accomplished by an application of a voltage between the second and first comb fingers, which causes rotation of the first comb fingers along with the rotating element back towards their initial position. The biasing force may be kept constant in this case. The capacitance between the second and first comb fingers is measured and used to monitor the angular motion of the rotating element. The measured capacitance can be further utilized in a feedback loop to control the angular position of the rotating element.
In another embodiment of the present invention, the combdrive device provides for a position sensor. A capacitance sensor measures a capacitance between the first and second comb fingers to monitor the angular position of the rotating element by way of the measured capacitance. A time-varying biasing force may be applied in this case to generate further rotation of the rotating element along with the first comb fingers in a predetermined manner. The angular position signal can be further fed to a feedback loop, so as to control the biasing force and hence the angular motion of the rotating element.
The present invention accommodates alternative position sensors comprised of gap closing electrodes, additional comb fingers, piezoresistive strain gauges, coils, magnets, piezoelectric sensors, optical sensors and combinations thereof.
The rotatable flexure may be a torsional flexure with cross-sections including a rectangular, I-shaped, or T-shaped cross-section, a cantilever-like flexure, serpentine flexure, a pin-and-staple type hinge, or any flexure, as one skilled in the art is capable of applying, to achieve rotation. A variety of biasing mechanisms may be employed to generate an initial angular displacement between the first and second comb fingers, including those that operate via pneumatic, thermal, magnetic principals, including coils that interact with an external magnetic field, electrostatic elements, such as gap closing electrodes, piezoelectric actuators and thermal bimorph actuators. Alternatively, the biasing element may be a mechanical, e.g., spring-loaded element, which may be incorporated into the rotatable flexure. In an alternative embodiment of the invention two sets of biased comb structures may be incorporated into a two-dimensional scanner. The scanner generally includes a rotatable gimbaled structure having a base, an outer frame, and an inner part. The outer frame may be attached to the base by a first pair of torsional flexures that allow the outer frame to rotate about a first axis. The inner part, hereby also known as the rotating element, may be attached to the outer frame by a second pair of torsional flexures that allow the inner part rotate about a second axis. The inner part may include a reflective surface such as a mirror. The scanner may include one or more electrostatic combdrives: a first combdrive positioned between the outer frame and the base, and a second combdrive positioned between the inner part and the outer frame. A biasing element, coupled to the outer frame, the inner part, or to both, causes initial angular displacements in the first and second combdrives. Either or both of the first and second combdrives may include one or more self-aligned comb structures.
Applying a voltage to either of the first and second combdrives causes the rotating element to undergo further rotation about either of the first and second axes. The biasing element may exert one or more constant forces on the rotating element. Moreover, the capacitances of the first and second combdrives may be measured to monitor and control the angular positions of the rotating element about the first and second axes respectively. Such a device may constitute a two-dimensional rotating actuator and use feedback from an alternative position sensor (e.g. gap closing electrodes, additional comb fingers, piezoresistive strain gauges, coils, magnets, piezoelectric sensors, optical sensors and combinations thereof) to control the angular position of the rotating element.
Alternatively, the biasing element may to exert one or more time-varying forces on the rotating element, causing it to undergo further rotation about either of the first and second axes. The capacitance between the first and second combdrives may be measured to monitor and control the angular positions of the rotating element about the first and second axes respectively. Such a device may provide for a two-dimensional actuator with a biaxial combdrive position sensor and use feedback from an alternative position sensor (e.g. gap closing electrodes, additional comb fingers, piezoresistive strain gauges, coils, magnets, piezoelectric sensors, optical sensors and combinations thereof) to control the angular position of the rotating element.
The first and second combdrives may be fabricated in a single layer of a substrate material which may comprise, but not be limited to, a combination of one or more of the following materials: single-crystalline silicon, poly-crystalline silicon, amorphous silicon, ceramic, silicon-oxide, silicon-nitride, single-crystalline silicon-germanium, polycrystalline silicon-germanium, or a metal (such as nickel, chromium, aluminum or gold). The rotating element may be made of the same materials. The rotating element may comprise a reflective or light deflective surface, thereby providing a bi-axial steering or scanning mirror. The light deflective surface may include a surface coating to allow light deflection along more than one simultaneous path. Either of the first and second flexures can be a torsional flexure with a cross-section including, but not limited to, rectangular, I-shaped, or T-shaped cross-section, a cantilever-like flexure, serpentine flexure, pin-and-staple type hinge flexure and mechanical or non-mechanical torsion providing means as one skilled in the art would be capable of applying. First and second axes may be typically orthogonal to each other, though they can also be oriented in other ways as dictated by practical applications. The biasing element may comprise, but not limited to one or more biasing elements of magnetic, thermal, electrostatic, or mechanical types.
One embodiment of the invention utilizes self-aligned combdrives. An important advantage of self-aligned rotating combdrive devices is that by fabricating the comb fingers in a single layer of a substrate, the first and second comb fingers may start from a substantially co-planar and interdigitated engagement, thereby substantially diminishing non-linear rotational effects that are often inherent in the prior art vertical combdrive actuators. Furthermore, if the first and second combdrives are defined by a single lithographic step, their alignment can be held to much tighter tolerances than in the prior art, providing for much more stable behavior than vertical combdrive actuators of the prior art. The performance of the rotating actuators and position sensors thus constructed is therefore more predictable than, and superior to, the prior art vertical combdrive devices.
It must be stated that the uni-axial and bi-axial rotating actuators and position sensors of the present invention can be employed in a broad range of applications, including, but not limited to, biomedical devices, optical devices for tracking and display, telecommunication devices such as fiber-optic switches, inertial sensors, and magnetic disk drives. For example, uni-axial and/or bi-axial rotating actuators employing reflective rotating elements can be used as steering mirrors to switch light between optical fibers in telecommunication applications. Arrays of such steering mirrors can be utilized to provide fiber-optic switches with very large port-counts. Use of the combdrives for sensing the angle of the mirrors in these applications is crucial since position sensing is needed for active and accurate control of the mirror angles, and pointing accuracy is the key to achieving low insertion losses in a fiber-optic switch.